The present invention relates to a controller for stabilizing the oscillation frequency of a semiconductor laser light source and improving the spectral linewidth thereof.
In heterodyne/coherent optical fiber communication, the difference between the oscillation frequency of a signal light source and that of a local oscillator light source needs to be kept stable because the frequency difference between them is detected and processed in an electric section as a beat signal. However, a semiconductor laser light source for such optical fiber communication is not good enough in the stability of the oscillation frequency of the light source and the spectral linewidth thereof. A procedure employing an external resonator has been so far used in order to stabilize the oscillation frequency and improve the spectral linewidth.
FIG. 2 shows a block diagram of a conventional semiconductor laser light source controller A including an external resonator 2, an optical divider 3, an optical frequency discriminator 4, a light receiver 5, and an automatic frequency control circuit 6. The external resonator 2 is for performing adjustment to alter an equivalent optical path length or phase for backward light L2 emitted backward from a semiconductor laser light source 1. Forward light L1 emitted foward from the light source 1 is divided into two lights L3 and L5 by the optical divider 3. The optical frequency discriminator 4 converts the change in the frequency or wavelength of the light L3 into that in the intensity of light L4. The light receiver 5 performs optoelectric conversion. The automatic frequency control circuit 6 regulates an injection current or the like for the semiconductor laser light source 1. The light L3 is subjected to optical frequency discrimination by the optical frequency discriminator 4 so that frequency-intensity converted light L4, the change in the intensity of which corresponds to that in the frequency of the light L3, is sent out from the discriminator 4. The light L5 is communication signal light, which is transmitted through a main line. Electric signals E1, E2, and E3 are generated by the light receiver 5 and auto-matic frequency control circuit 6. The change in the frequency of the light L3 sent out from the optical divider 3 is converted into that in the intensity of frequency-intensity converted light L4 by the optical frequency discriminator 4. The change in the intensity of frequency-intensity converted light L4 is converted into that in the intensity of the electric signal E1 by the light receiver 5.
Since the oscillation frequency of the semi-conductor laser light source 1 generally depends on the driving current for the light source, the electric signal E1 is supplied to the automatic frequency control circuit 6 in order to perform the feedback control of the driving current to stabilize the oscillation frequency. The length or phase of the external resonator 2 is altered in order to improve the spectral linewidth of the semiconductor laser light source 1. The oscillation frequency can be also stabilized by causing the electric signal E2 to alter the phase of return light L6 sent out from the external resonator 2 to the semiconductor laser light source 1. The external resonator 2 is made of a 100% reflection mirror, a diffraction grating or the like. Since the divided light L3 of the forward light L1 needs to be branched to the optical frequency discriminator 4, the forward light L1 undergoes a loss at a certain ratio to the communication signal light L5. For that reason, the conventional semiconductor laser light source controller has a problem that the interval between the repeaters in the main line is shortened. Therefore, it has been strongly desired to provide a device for performing the discrimination of the oscillation frequency of the semiconductor laser light source without causing the forward light to undergo the loss.